Low-salinity (LS) waterflooding has been a topic of substantial recent interest in the petroleum industry. Studies have shown that LS brine injection can increase oil production relative to high-salinity (HS) brine injection, but contradictory results have also been reported and a mechanistic explanation of these findings remains elusive. We have recently developed a pore-scale model of LS brine injection in uniformly wetted networks (Watson et al. in Transp Porous Med 118:201–223, 2017), and we extend this approach here to investigate the low-salinity effect (LSE) in heterogeneously wetted media. We couple a steady-state fluid displacement model to an innovative tracer algorithm and track the evolving salinity front as oil and HS brine are displaced from the network. The wettability of the pore structure is locally modified where water salinity falls below a critical threshold, and simulations show that this can have significant consequences for oil recovery. Our results demonstrate that, for heterogeneously wetted networks, the oil-wet (OW) pores are the only viable source of incremental oil by LS brine injection. Moreover, we show that a LS-induced increase in the displaced OW pore fraction is a necessary, but not sufficient, condition to guarantee additional oil production. Simulations further suggest that the initial OW pore fraction, the average network connectivity and the initial HS brine saturation are factors that can determine the extent of incremental oil recovery following LS brine injection. This study clearly highlights that the mechanisms of the LSE can be markedly different in uniformly wetted and in non-uniformly wetted porous media.

中文翻译：

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非均质润湿多孔介质中低盐度效应的机械孔隙尺度分析

低盐度 (LS) 水驱一直是石油工业最近非常感兴趣的话题。研究表明，相对于高盐度 (HS) 盐水注入，LS 盐水注入可以增加石油产量，但也报道了相互矛盾的结果，对这些发现的机械解释仍然难以捉摸。我们最近开发了均匀润湿网络中 LS 盐水注入的孔隙尺度模型（Watson 等人在 Transp Porous Med 118:201–223, 2017 中），我们在这里扩展了这种方法以研究低盐度效应（LSE ) 在异质润湿介质中。我们将稳态流体置换模型与创新的示踪算法相结合，并随着石油和 HS 卤水从网络中置换出来，跟踪不断演变的盐度前沿。孔隙结构的润湿性在水盐度低于临界阈值的地方被局部改变，模拟表明这会对石油采收产生重大影响。我们的结果表明，对于非均质润湿网络，亲油 (OW) 孔隙是 LS 盐水注入增量油的唯一可行来源。此外，我们表明，LS 引起的置换 OW 孔隙分数的增加是保证额外石油产量的必要条件，但不是充分条件。模拟进一步表明，初始 OW 孔隙分数、平均网络连通性和初始 HS 盐水饱和度是可以确定 LS 盐水注入后增油程度的因素。